Contacts between metal and semiconductor active areas are currently improved by forming a refractory metal silicide at the interface between the two materials. Titanium silicide is the most commonly used silicide. Titanium is deposited or sputtered on the semiconductor active area and annealed to form titanium silicide. This provides a good, low resistance contact.
Borophosphosilicate glass (BPSG) is used as a device insulator, surrounding contact areas. Recently, in order to get BPSG to reflow at lower temperatures, the amount of dopants in the BPSG film is being increased. It is desirable to lower the reflow temperature of BPSG to avoid diffusion of dopants into undesired areas during high temperature steps. Normally, reflow temperatures exceed those required during annealing process steps. However, the addition of dopants causes BPSG to reflow even at the temperatures required for the titanium anneal process. This is undesirable because it causes titanium to buckle, resulting in degradation of contacts by increasing their resistance. This is counterproductive to the main reason for forming titanium silicide at semiconductor/metal interfaces--to improve contacts by lowering their resistance.
The addition of a refractory metal nitride layer at the surface of the refractory metal silicide provides both a barrier to diffusion into the contact, and helps with the adhesion of the refractory metal silicide to the metal, which may comprise tungsten, aluminum, and similar conductive metals. Conventionally, such layers are formed by annealing in a nitrogen-containing ambient simultaneously with forming refractory metal silicide because it is important that a barrier nitride layer form simultaneously with titanium silicide, so that both can be formed in one processing step. Typically, titanium silicide is used for the refractory metal silicide and titanium nitride is used for the barrier layer.
As devices are becoming smaller, there is a need for lowering the temperature at which the refractory metal anneal occurs when forming refractory metal silicide at semiconductor/metal interfaces. In smaller devices, the acceptable amount of thermal-induced dopant diffusion is lower. There is a further need for lowering the temperature at which refractory metal nitride is formed in order that the refractory metal nitride layer can be formed simultaneously with the refractory metal silicide layer. It is paramount that the temperature of these anneals be lowered so that low temperature reflow doped oxide insulator layers can be used.